This invention relates to a corrosion measuring sensor. More particularly, this invention relates to a corrosion measuring probe in combination with a predesignated bridge circuit. Still more particularly, this invention relates to a corrosion measuring sensor sized to be compatible with a predesignated bridge circuit over a plurality of ranges for a given material and anticipated corrosion rate.
Early methods of determining the corrosive or other effects of fluids involved complicated efforts both in the field and laboratory. Thus, in some circumstances, fluid samples from field locations were taken to a laboratory for testing. By another method, a piece of test metal, sometimes referred to as a coupon, was immersed in the fluid for a recorded time duration at a field location, then removed from the fluid and subjected to a laboratory determination of the changes which occurred in the coupon during its immersion. From such data, the corrosivity of the fluid could be estimated.
More recently, corrosion measuring probes of various designs have been fixedly installed in the walls of processing equipment so that the somewhat fragile and delicate metal sensing elements of the probe project into the fluid. The metal of the sensing elements is preferably the same as the metal from which the process equipment is made, and preferably has the same electrical resistance characteristic, so that the corrosion of the sensing element, because of its increased resistance resulting from corrosion, is indicative of the extent of corrosion of the similar metal within the process equipment. A representative example of such a probe is described in U.S. Pat. No. 3,320,570 to E. B. Lied, Jr.
Suitable electrical instruments have been connected to such probes to measure and indicate the extent of the increase in resistance of the sensing element, using primarily null-balance and servo-balance techniques. Such electrical instruments, or at least substantial parts thereof, have heretofore been separate from the probes, but with plug facilities by means of which one set of instruments could be separately plugged to different probes to determine the resistance values of their sensing elements. The prior art contains a wide variety of bridge circuits for use in combination with probes of various designs.
While the probe and circuit combination of the present invention may be generally applied as described above, the invention has a special adaptability which includes at least some of the electrical circuitry within and as parts of the unitary probe structure, or permanently installed in the immediate vicinity thereof.
It is a feature of this invention to provide a simplified, unitary probe structure in which the components of the probe are more readily machined, easily assembled and sealwelded. In addition, this invention provides a probe in which the sensing element is so constituted that, for either assembly or repair, a sensing element can easily be welded into place without causing any material change in the resistance characteristics of the sensing element. This result arises from an arrangement in which the weld on the sensing element is located on a relatively massive hub portion thereof, at a substantial distance from the relatively low resistance measuring portion of the probe, forming a positive, yet readily replaceable leak-proof seal suitable for use with high temperature and high pressure processes. The relatively massive hub portion of the sensing element substantially absorbs the added weld metal with negligible change in calibration and diffuses the welding heat so that the low resistance of the sensing element is not significantly affected. The massive hub portion also serves as a galvanic-action inhibitor in that it connects parts which often are of different metals and which, except for the presence of the massive hub portion, would undesirably give rise to galvanic-action between these metals, possibly affecting the resistance value of the sensing portion.
Another aspect of the invention provides an electrically resistive inner-end corrosion measuring portion having a wall of medium or heavy thickness which will not provide an erroneous high corrosion indication during the latter life of any probe which could be subject to pitting attack. Further, each test element is made strong and sturdy, so as not to fail by metal fatigue in fast moving streams even though the test element or sensing element may not be protected by an outer shield. In addition, each test element is designed so that the effects of rapidly varying process temperatures are almost completely nullified for general operation conditions, to the degree that normal fluctuating temperatures will not cause significant difficulties in accurately determining corrosion measurements. The probe construction also includes features which eliminate the common sources of electrical noise, crosstalk, hysteresis effect, and errors due to residual or thermocouple effects, particularly in instances where accuracy is required during corrosion monitoring in services involving elevated process temperatures. When used with compatible corrosion bridge measuring circuits, test sensors of the invention have a long life compared to the probes made by prior art techniques, without any loss of corrosion measuring sensitivity.
Still another aspect of the invention relates to a corrosion bridge measuring circuit which includes circuit means having parameters selected to be compatible with sensor design. The activity, or voltage output, of the bridge circuit is selected to accommodate a sensor which is sized as a function of the desired tracking range of the sensor and its electrical resistivity. The tracking rate is determined from the anticipated corrosion rate of corrosive medium, while the electrical resistivity is governed by the material from which the sensor is made. By sizing a probe to produce resistance values over an anticipated range as a function of the electrical resistivity of a given material and the anticipated corrosion range, the sensor and circuit combination can be utilized for a substantially longer period of time than can be utilized for a substantially longer period of time than sensors of the prior art. By developing a plurality of predesignated bridges, a table can be constructed to determine for an anticipated corrosion rate and a given resistivity of material, the appropriate bridge for which the probe can be used.
The sensitivity of the circuit to incremental corrosion measurement is greatest at the maximum activity or voltage output of the bridge, although the wall thickness of the sensor at depletion is generally thinner than may be desirable for certain circumstances. On the other hand, the sensitivity of the circuit is less at a lesser activity and the wall thickness of the sensor is greater.
The bridge circuit of the invention provides for a half bridge, which is half active and which possesses a relatively low corrosion bridge gage factor to produce a given output for a fully active bridge. The "gage factor" for these purposes may be considered to be the ratio of the resistance of the half active leg for a new sensor to the resistance of the half active leg at depletion. For a fully active bridge, the gage factor is at its maximum, and for less than a maximum bridge output voltage, the bridge gage factor has a lesser value. Therefore, the maximum gage factor for a bridge can be used to determine the physical dimensions of a sensing element which will produce a fully active bridge at maximum sensitivity. By maintaining bridge gage factors extremely low, the physical result is that the sensors will have a relatively large cross-sectional area for the corrosion measuring portion which will permit the sensor to be fabricated by machine operations.
In the invention, only a portion of the active arm of the bridge is exposed to the corrosive medium and the remaining portion of that leg must be covered for attachment of leads and contacts. By using bridges having low gage factors, a greater portion of the active leg may be covered and only a minor portion need be exposed to activity by corrosion to obtain the maximum bridge output. This feature also permits the construction of a sensor having a relatively large cross-sectional area and reduces the effect of temperature fluctuations on the output signal.
These and other objects and advantages of the system according to the invention will become apparent from the review of the detailed description of the invention which follows taken in conjunction with the accompanying drawing.